Environmental Engineering Reference
In-Depth Information
palaeomagnetic direction and radiometric age, either side of the axis underpin our
reconstruction and timing of ocean evolution (see Figure 10.6). Lying beneath an average
ocean depth of 3 km, ridge axes are also important minerogenic centres where
hydrothermal processes exchange minerals at the ocean-lithosphere interface (see
Chapter 12).
Abyssal plains
occupy most of the deep ocean floor between the ridges and trenches,
covering 42 per cent of the total area, at an average depth of 5-6 km. They are
Figure 11.4
Age of the ocean floor.
Source: After Scotese et al. (1988).
floored by cool, older oceanic crust which has subsided into the lithosphere beyond the
spreading ridges and are the flattest places on Earth, broken only by submarine plateaux
and
seamounts
. The latter are distributed randomly and form away from ridges, although
they may be associated with their positive thermal anomalies. Elsewhere, seamount
chains form as ocean crust migrates over fixed hot spots. Islands such as Hawaii appear
where they break surface but most are submarine
guyots
. These are summits levelled by
marine planation or post-eruptive subsidence and many provides attachment sites for
coral reefs in shallow, clear-water tropical seas. A thick carpet of
pelagic sediments
provides the stark contrast between seamounts, sloping at 15°-25°, and abyssal plains at
less than 0·1°. They are derived from minero-biogenic sediment sources by rain-out or
solid precipitation and infill bedrock depressions.
